At present, companies are faced with very important and challenging ecological issues such as reducing industrial waste, sorting out industrial waste and recycling or avoiding discharge of industrial waste into the nature. One type of industrial waste comprises various fluids containing contaminants.
These fluids have different denominations, such as for instance sewage water, drainage, effluent, etc., but in the description to follow, fluids, like for example water, chemicals, etc. comprising materials which are contaminants are called wastewater. Such wastewater is filtered using very expensive filtration apparatuses to remove the contaminants therein. The resulted clean water is recycled, whereas the separated contaminants or the residues which could not be filtered are disposed of as industrial waste. In particular, water is filtered to a level of purity which meets the environmental standards and is then discharged into the nature, in rivers, seas, or the like, or is otherwise recycled.
However, the usage of these filtration apparatuses is extremely difficult and becomes an environmental issue in itself due to a high running cost and equipment cost, etc. of the filtration process, etc.
As can be understood from the above, technology and techniques for wastewater treatment raise an important issue in terms of environmental pollution and recycling and therefore a system with a low initial cost and low running cost is promptly required.
An example of wastewater treatment in the semiconductor field is described in the following. When a plate-like metal, semiconductor or ceramic, etc. is polished or ground, a liquid such as water, etc. is showered on the polishing (grinding) jig and the plate due to considerations such as prevention of temperature rise of the polishing (grinding) jig, etc. caused by friction, improvement of lubricating property, prevention of polishing waste or cutting waste adherence, etc. to the plate.
In more detail, pure water is caused to flow during the process of dicing or backgrinding a semiconductor wafer comprising a plate-like semiconductor material. In a dicing apparatus, a flow of pure water is provided on the semiconductor wafer or pure water is showered by means of a discharge nozzle such that the blade is exposed to the pure water in order to prevent a rise in the temperature of the dicing blade and prevent adherence of dicing waste to wafer. Pure water is also used due to similar considerations in a process of thinning the wafer by backgrinding.
The wastewater containing polishing waste or grinding waste discharged from the above dicing apparatus or backgrinding apparatus is filtered and the clean water obtained thereby is returned into nature or is recycled, whereas the concentrated wastewater is recovered.
At present, two methods are employed in the semiconductor manufacturing process for the treatment of wastewater containing contaminants (waste) comprising mainly Si: the flocculation method and a method combining filter filtration and a centrifugal separator.
In the flocculation method, PAC (polychlorinated aluminum) or Al2(SO4)3 (aluminum sulfate), or the like is mixed in the wastewater as flocculant to generate a reaction with the Si and filtration is carried out by removing the reaction.
In the method combining filter filtration and a centrifugal separator, after the wastewater is filtered, the concentrated wastewater is fed to the centrifugal separator and silicon waste is collected as sludge, whereas the clean water resulted from the filtration process is discharged into the nature or is recycled.
For instance, wastewater generated in the dicing process is collected in a raw water tank 201 and is fed to a filtration apparatus 203 via pump 202 as shown in FIG. 16. The filtration apparatus 203 is provided with a ceramic and an organic filter F and the water filtered thereby is fed to a recycled water tank 205 via a pipe 204, to be further recycled or discharged into the nature.
The filtration apparatus 203 is periodically washed due to clogging of the filter F. The filter F is back-washed with the water inside the recycled water tank 205 by closing valve B1 provided in the raw water tank 201 and opening valve B3 and valve B2 which feed washing water from the raw water tank 201. The wastewater thus generated and having a high concentration of Si waste mixed therein is returned to the raw water tank 201. The concentrated water in the concentrated water tank 206 is fed to the centrifugal separator 209 by means of a pump 208 and the centrifugal separator 209 separates the sludge and the separate liquid. The sludge comprising Si waste is collected in a sludge recovery tank 210, whereas the separate liquid is collected in a separate liquid tank 211. The wastewater in the separate liquid tank 211 containing the separate liquid is fed to the raw water tank 201 by means of a pump 212.
These methods are also employed when collecting waste generated during polishing and grinding of, for instance, solids mainly comprising metal materials such as Cu, Fe, Al, etc., or solids comprising inorganic materials such as plates, ceramic, etc., or plates, etc.
On the one hand, the CMP (Chemical-Mechanical Polishing) method has emerged as a novel semiconductor process technology. The CMP technology was brought about by the achievement of plane insulating film devices and by the implementation of embedded structures comprising a material different from the material of the substrate.
Plane insulating film devices are formed by forming a highly accurate micropattern using the lithography technology. Together with the Si wafer attachment technology, etc., this achievement provides the potential for the implementation of three-dimensional ICs.
With respect to the implementation of embedded structures, conventionally, a technology is used where tungsten (W) is embedded in the multilayer wiring of an IC. Here, W is embedded in the grooves of an interlayer film by the CVD method and a surface thereof is planarized by etch-back. However, recently, the CMP method is used in the planarization process. The embedding technology can be applied in the Damascene process and elements separation. The CMP technology and its applications are described in detail in “CMP Science” published by Science Forum.
Next, the mechanism of the CMP technology is described. As shown in FIG. 17, a semiconductor wafer 252 is placed on an abrasive cloth 251 provided on a rotating table 250 and the uneven surface of the wafer 252 is leveled by lapping, polishing and chemical etching while causing a polishing material (slurry) 253 to flow. Planarization is obtained by a mechanical polishing process between a chemical reaction of a solvent included in the polishing material 253 and a polishing abrasive coating included in the abrasive cloth and the polishing material. Foamed polyurethane, non-woven cloth, etc. can be used as the abrasive cloth 251, whereas the polishing material is a material comprising polishing abrasive coating such as silica, alumina, etc. mixed with water comprising a pH adjuster, and is generally known as slurry. Lapping is carried out by applying a constant pressure while rotating the wafer 252 in the abrasive coat and causing the slurry 253 to flow. A dresser 254 has the function of maintaining the polishing capabilities of the abrasive cloth 251 and always keeps a surface of the abrasive cloth 251 in a dressed state. Motors 202, 208 and 212 and belts 255, 256 and 257 are also provided.
The above-described mechanism is constructed as a system, as shown in FIG. 18. This system can be divided in a wafer cassette, loading/unloading station 260, a wafer reprint mechanical section 261, a polishing mechanical section 262 as described with reference to FIG. 17, a wafer washing mechanical section 263 and a control system controlling all these elements.
First, the cassette 264 comprising wafers is placed in the wafer cassette loading/unloading station 260 and a wafer inside the cassette 264 is removed. Next, the wafer reprint mechanical section 261 holds the wafer with, for instance, a manipulator 265 and places it on the rotating table 250 provided in the polishing mechanical section 262. Planarization of the wafer is carried out using the CMP technology. When the planarization process is finished, the wafer is fed to the washing mechanical section 263 by the manipulator 266 and is washed in order to wash away the slurry. The washed wafer is housed in a wafer cassette 266.
The amount of slurry used in one process is, for instance, around 500 cc to 1 liter/wafer. Also, pure water is caused to flow in the polishing mechanical section 262 and wafer washing mechanical section 263. Thus, the total amount of wastewater, at drainage, discharged in one planarization process is around 5 liters to 10 liters/wafer. For example, in case of a 3-layered metal, the planarization process is carried out about 7 times for metal planarization and interlayer dielectric film planarization, accordingly, the wastewater amount discharged until one wafer is completed is 5 to 10 liters multiplied by 7. Thus, it can be understood that by using the CMP apparatus, the amount of slurry diluted by pure water and discharged is quite large. The wastewater is then treated by the flocculation method.
However, with the flocculation method, chemicals are injected as flocculants. Nevertheless, it is extremely difficult to specify the amount of chemicals that fully react and a large amount of chemicals which do not react is left. By contrast, if the amount of chemicals is low, not all of the contaminants are coagulated and are left un-separated. In particular, if the amount of the chemicals is large, chemicals are left in the clear supernatant liquid and therefore, because chemicals remain in the filtered liquid, reuse thereof is impossible at when further chemical reactions need to be carried out.
Flocks which are reactions of the chemicals and contaminants are generated by the suspension of algae. The formation of flocks requires very strict pH conditions, an agitator, a pH measurement device, a flocculating agent implantation device, and a controlling device, for controlling all of these elements, etc. Also, in order to stabilize and cause precipitation of the flocks, a huge precipitation tank is required. For instance, for a wastewater processing capability of 3 m3/1 h, a tank having about 3 meters in diameter and about 4 meters in depth (around 15 tones) is needed, so that the entire system becomes a huge system requiring a compound of about 11 meters by 11 meters.
However, there are also flocks that keep floating without precipitating in the precipitation tank. It is therefore possible that they are discharged to the exterior and recovery thereof is very difficult. Due to size considerations, initial cost of this system is expensive, reuse of water is difficult and the running cost generated by the use of chemicals is very expensive.
In a method combining filter filtration (5 m3/1 h) and a centrifugal separator, as shown in FIG. 16, a filter F (it is called a UF module and comprises polysulfone fiber or ceramic filter) is used in the filtration apparatus 203 which enables reuse of water. Four filters F are attached in the filtration apparatus 203, one filter costing around 50000 yen and having a life span of no more than a year. Due to the fact that filter F is a pressurized filtration method, the filter clogs and the load to the motor of pump 202 increases thus requiring a high-capacity pump 202. Moreover, ⅔ of the wastewater passing through the filter F are returned into the raw water tank 201 and because the wastewater containing removables is supplied using pump 202, the inner walls of the pump 202 are chipped and the life span of the pump 202 is extremely short.
To summarize, running cost, including extremely high power consumption by the motor, high costs associated with replacement of the pump P and filter F, becomes extremely high.
Moreover, with the CMP method, in the dicing process, a very large amount of wastewater is discharged. Colloid slurry is distributed in the fluid, but due to a Brownian motion, they do not precipitate. Particles of the abrasive coating mixed in the slurry have a diameter of 10 thru 200 nm, in other words, they are very fine particles. Consequently, when the slurry comprising very fine abrasive coating is filtered through the filter, particles enter the holes provided in the filter causing clogging. The filter clogs very frequently so that it is impossible to filter a large amount of wastewater.
As can be understood from the above description, in order to remove as much as possible all materials that may cause damage to the environment and to recycle filtered fluid and removables separated in the filtration process, the wastewater filtration system becomes a huge system due to various additional apparatuses, thus triggering extremely high initial cost and running cost. Accordingly, wastewater treatment apparatuses until now, could not be easily installed and used for wastewater treatment.
When using a self-generated film (pre-coat filter) to filter the wastewater, control of the pump carrying out filtration is very difficult.
Furthermore, in order to recover the removables precipitated at the bottom of the tank, filtration process is temporarily halted in order to discharge the fluid inside the tank. This led to a reduction of filtration efficiency.
Also, when filtration is carried out in a filtration apparatus having a self-generated filter, there is no appropriate method of removing the self-generated film which clogged.